The present invention relates to a demagnetizing device for eliminating remnant magnetization of a magnetic recording and reproducing head and more particularly to a compact hand-held demagnetizing device.
Magnetic recording and reproducing head is composed of a magnetic material of a high magnetic permeability such as ferrites, permalloy and the like but tends to leave a small remnant magnetization under the influence of the magnetic flux from magnetic tapes, recording current, bias current and the like. The remnant magnetization can become a source of noise when sounds are reproduced from a recording medium such as a magnetic tape, resulting in reproduced sounds of low quality. To avoid this detrimental effect, it is necessary to effect periodical demagnetization of the magnetic head. In the past, it was a practice to utilize a demagnetizing apparatus which is adapted to generate an alternating current of a sufficiently high intensity and usually of a commercial frequency. The demagnetizing apparatus was brought to a location close to the magnetic head to be erased and then gradually moved off from the magnetic head, thereby to gradually attenuate the alternating magnetic field.
However, the demagnetizing or erasing head of this type is large in size and requires manual movement of the demagnetizing apparatus. This means that the demagnetizing operation requires skill and a relatively long period of time and due to the vibration caused by the commercial frequency unexpected troubles can be caused. In order to overcome these disadvantages, the inventor proposed a compact demagnetizing circuit for generating an attenuating alternating magnetic field by utilizing charging or discharging properties of a capacitor. The demagnetizing circuit is composed of a charging portion for charging the capacitor and an oscillator portion controlled by the discharge of the capacitor to generate a decreasing alternating electric current which is applied to a drive coil for a demagnetizing or erasing head.
However, the demagnetizing device of this type cannot indicate the start and the completion of the demagnetizing operation. Moreover, it is desirable to know the time when the capacitor is charged at a level exceeding a predetermined potential necessary for generating a decreasing alternating demagnetizing magnetic field whereby the operator is notified that he can start the demagnetizing operation.